1. Field of the Invention
The present invention relates generally to plug heads which plug desired portions of pipes through which fluid such as gas, oil, water, etc., flows at high pressure and, more particularly, to a double plug head which is used when plugging a desired portion of a pipe to conduct maintenance of the pipe, for example replacement of a valve, installation or movement of the pipe, replacement of an old pipe with a new one, etc., and which is configured such that the pipe is primarily plugged by a rubber packing provided around the plug head and is secondarily plugged by a rubber tube which can be expanded by compressed fluid injected thereinto, so that a gap between the inner surface of the pipe and the rubber tube can be completely plugged, thus reliably preventing gas or the like from leaking through the plug head.
2. Description of Related Art
Generally, city gas is used as fuel in most households, factories, stores and so on, so that a great number of gas pipes are laid under the ground in a city or village in the shape of a cobweb. In addition, various kinds of fluid supply pipes, such as pipes for transporting oil, hot-water supply pipes, service water pipes, etc., are laid under the ground. In the following explanation, a gas pipe will be used as a representative example of such a pipe.
In such a pipe, when maintenance of a pipe, for example replacement of a valve, installation or movement of the pipe, replacement of an old pipe with a new one, etc., is conducted, it is required to plug a desired portion of the pipe such that gas cannot flow through the pipe or that it is bypassed. In this case, the flow of gas through the main pipe is blocked in such a way as to insert a plug head into the main pipe to be plugged using a plugging machine.
FIG. 1 illustrates a conventional plug head which is in a state of being inserted into a main pipe 1 using a plugging machine. To plug the main pipe 1, a branch fitting 4 and a control valve 3 are provided on the main pipe 1. Thereafter, a branch pipe 2 is connected to the branch fitting 4. Subsequently, a plugging machine 10 is coupled to the branch pipe 2.
A guide 12 and a lever unit 13 are connected to a piston rod 11 of the plugging machine 10. The lever unit 13 is provided with a mounting plate 14. A rubber packing 21′ and a roller support 22 are coupled to the mounting plate 14. Rollers 22a, 22b and 22c are provided on the roller support 22.
When the piston rod 11 of the plugging machine 10 is extended downwards, the frontmost roller 22a comes into contact with the bottom of the inner surface of the main pipe 1 and then the lever unit 13 rotates. Thereafter, the second roller 22b comes into contact with the bottom of the main pipe 1. Finally, the lever unit 13 rotates 90° and the third roller 22c comes into contact with the bottom of the inner surface of the main pipe 1.
As such, when the lever unit 13 rotates 90°, the rubber packing 21′ seals the main pipe 1, thus blocking the flow of gas. In this state in which the flow of gas is stopped, maintenance of the pipe is conducted at a downstream side of the main pipe 1 with respect to the plug head.
However, typically, the inner surface of the main pipe 1 is uneven because it rusts due to having been used for a long period of time or because of impurities which are applied thereto. Hereby, a fine gap may be formed between the rubber packing 21′ and the inner surface of the main pipe 1, resulting in gas leaking. Particularly, such a leakage of gas makes maintenance work on the pipe dangerous.